This invention relates to improved functional fluid compositions containing erosion inhibitors. This invention further relates to phosphate ester-based functional fluids, particularly phosphate ester-based hydraulic fluids, containing the erosion inhibitors of this invention.
In the past, functional fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids, and as a filter medium for air-conditioning systems. Phosphate ester-based functional fluids have been recognized for some time as advantageous for use as the power transmission medium in hydraulic systems. Such systems include recoil mechanisms, fluid-drive power transmissions, and aircraft hydraulic systems. Hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. Phosphate ester-based fluids find particular utility in aircraft hydraulic fluids because of their special properties which include high viscosity index, low pour point, high lubricity, low toxicity, low density and low flammability. Thus, for many years, numerous types of aircraft, particularly commercial jet aircraft, have used phosphate ester-based fluids in their hydraulic systems. Among the most important requirements of an aircraft hydraulic fluid is that it be stable against oxidative and hydrolytic degradation at elevated temperatures.
In addition, functional fluids for use in aircraft hydraulic systems must be capable of performing in the hydraulic system over an extended period of time without causing significant damage or functional impairment to the various conduits, valves, pumps, and the like, through which the functional fluid flows in the course of such use. Damage caused by functional fluids contacting valves and other members has been attributed to streaming current induced corrosion, hereinafter referred to as erosion, by the environment in contact with the functional fluid in a hydraulic system.
The hydraulic systems of a typical modem aircraft contain a fluid reservoir, fluid lines and numerous hydraulic valves which actuate various moving parts of the aircraft such as the wing flaps, ailerons, rudder and landing gear. In order to function as precise control mechanisms, these valves often contain passages or orifices having clearances on the order of a few thousandths of an inch or less through which the hydraulic fluid must pass. In a number of instances, valve orifices have been found to be substantially eroded by the flow of hydraulic fluid. Erosion increases the size of the passage and reduces below tolerable limits the ability of the valve to serve as a precision control device. For example, aircraft have experienced slow response of flight controls as a result of valve erosion. Thus, phosphate ester-based aircraft hydraulic fluids require use of an erosion inhibitor, i.e. a functional fluid additive which prevents or inhibits the erosion of hydraulic system valves. Other additives which perform special functions such as hydrolysis inhibition, viscosity index improvement and foam inhibition are also frequently present in such hydraulic fluid. For example, epoxides are utilized commonly in phosphate ester-based hydraulic fluids to stabilize the phosphate ester.
Current commercial phosphate ester-based aircraft hydraulic fluids such as Skydrol® LD-4 aviation hydraulic fluid and Skydrol® 5 aviation hydraulic fluid, both available from Solutia Inc., successfully utilize alkali metal salts of perfluoroalkyl sulfonic acids, e.g. Fluorad™ FC-98 of 3M Company, as erosion inhibitors. It would be desirable to have alternative erosion inhibitors available for use in phosphate ester-based aircraft hydraulic fluids. New erosion inhibitors for use in phosphate ester-based aircraft hydraulic fluids have now been discovered.